1. Field of the Invention
The present invention relates to a throttle valve control device for controlling an air-intake passage.
2. Related Art
U.S. Pat. No. 5,287,835 discloses a throttle valve control device for controlling an air-intake passage in which the throttle valve is driven by a torque motor. In order to increase the response speed of the throttle valve, a return spring for biasing the throttle valve in one direction is omitted.
A drawing of U.S. Pat. No. 5,287,835, shows a stator core of an actuator with no slot on the inner periphery surrounding a rotor. However, the drawing is only a diagrammatical view and there is no description on the inner periphery of the stator core in the specification. A stator core illustrated in another drawing has a separated N-pole stator section and an S-pole stator section, and slots or cut portions are formed between two sections. If the rotor is equipped with a permanent magnet made of rare-earth metal such as neodyum, samarium or cobalt, distribution of magnetic-flux density in the stator becomes more uneven than the distribution of magnet-flux density caused by the rotor equipped with a ferrite magnet. Accordingly, a large detent torque is applied to the rotor when the driving coil is turned off.
In an actuator which has a return spring for returning the rotor to the fully closed position to prevent the throttle valve from opening if the current control device of the torque motor becomes out of order, a large spring force is necessary to return the rotor to the fully closed position against the detent torque if there is a slot on the inner periphery of the stator surrounding the rotor. That is, large electro-magnetic force and, accordingly, a large-sized torque motor are necessary to drive the throttle valve against the spring force.
Although it is desired that the permanent magnet for magnetic poles on the rotor is magnetized in the radial directions, cracks may be formed during the step of cooling the permanent magnet if the magnetic poles of the permanent magnet are formed of a large number of sintered radially-magnetized particles. Thus, the yield rate decreases and the production cost increases.